Field of the Invention
The present invention relates to a process for producing hematite in which low-sulfur-grade hematite usable as, for example, an iron-making raw material is produced from leached residue generated when a nickel oxide ore is leached under pressure, and the hematite. The present application claims priority on the basis of Japanese Patent Application No. JP 2013-126805 filed on Jun. 17, 2013 in Japan, and this application is incorporated herein by reference.
Description of Related Art
A nickel oxide ore contains various components such as iron, cobalt and manganese in addition to nickel. In smelting for recovering nickel from the nickel oxide ore, a process called fire smelting has been often used in which an oxide ore is placed in a furnace and roasted together with a reducing agent.
In fire smelting, components that are not to be recovered, such as iron, manganese, aluminum and magnesium, are effectively separated from nickel and cobalt as a slug.
In recent years, a hydrometallurgical process called a HPAL process has also been used in which a low-grade nickel oxide ore that contains only about 1 to 2% by mass of nickel and cannot be economically smelted by the fire smelting process is placed in a pressurized vessel together with sulfuric acid, valuable metals such as nickel and cobalt are leached into the sulfuric acid solution under an atmosphere with a high temperature of about 250° C. and a high pressure, and separated from leached residue.
As described in, for example, Patent Literature 1, nickel and cobalt leached in the HPAL process are solid-liquid-separated from a slurry while remaining free acids are neutralized by adding a neutralizing agent, and a neutralizing agent is then added to separate the nickel and cobalt from impurities. Further, a sulfurizing agent is added to the leached nickel and cobalt to recover the nickel and cobalt in the form of a sulfide as an intermediate raw material, so that the nickel and cobalt are separated from impurity components such as aluminum, manganese and magnesium which remain in the sulfurized solution and are not to be recovered. The sulfurized solution containing these impurity components is then neutralized by adding a neutralizing agent thereto, and thereby forms waste water precipitates composed of the impurity components. The resulting waste water precipitates are mixed with leached residue, or individually deposited on a tailing dam to be treated.
On the other hand, the smelting process using a wet process has the problem that a larger amount of leached residue are generated with respect to the same nickel production amount because the nickel grade is low. The majority of leached residue are mainly composed of iron, and exist in the form of iron oxide (Fe2O3) called hematite particularly in the case of the above-mentioned HPAL process. Hematite is a type of iron ore, and therefore it is preferred that leached residue in the HPAL process are essentially provided as an ironmaking raw material to effectively use a resource and reduce the amount of wastes.
However, leached residue obtained in the HPAL process have not used because they are considered unsuitable for ironmaking applications.
This is because a large amount of a neutralizing agent is required for a neutralizing free acids sticking in a leachate in the case of practical operations for recovering nickel from a nickel oxide ore on an industrial scale. Specifically, in neutralization of free acids sticking in a leachate, a calcium-based neutralizing agent such as slaked lime or limestone, which can be easily and relatively inexpensively acquired on an industrial scale, is often used, but when the calcium-based neutralizing agent is used, calcium sulfate (gypsum) generated in neutralization also coexists as precipitates in leached residue, and therefore the grades of sulfur and calcium coexisting in hematite increase.
Particularly, when leached residue include sulfur, and are used as an ironmaking raw material in an ironmaking process, the sulfur may be discharged as a SO2 gas to the surroundings to cause destruction of the environment in the case of a blast furnace which does not have effective desulfurization equipment. Accordingly, specifically it has been required to reduce the content of sulfur in hematite to approximately 1% or less. Existence of calcium in iron steel may also affect the quality, and therefore the content of calcium is preferably low.
Accordingly, it may be practical to use a non-calcium-based neutralizing agent, for example sodium hydroxide. However, a neutralizing agent such as sodium hydroxide is not suitable for practical use in terms of cost when considering the industrial use scale.
Thus, a process has been devised in which a salt having a high solubility is used as a neutralizing agent to prevent a neutralized product from being caught in leached residue. Specifically, magnesium salts such as magnesium oxide and magnesium hydroxide are suitable for this use purpose. Further, there is a large amount of magnesium around or in a nickel oxide ore, and accordingly magnesium may be inexpensively and stably supplied.
Specifically, for example, Patent Literature 2 describes one of processes for recovering magnesium from a solution sent to a waste water treatment. Patent Literature 2 suggests a process for recovering magnesium oxide from a source of magnesium sulfate, the process including the steps of: providing a source of solution-state magnesium sulfate which is obtained from a part of a process related to leaching of a metal-containing ore or smelted ore; transforming the solution-state magnesium sulfate into solid magnesium sulfate; bringing the solid magnesium sulfate into contact with elemental sulfur in a reducing atmosphere; and recovering magnesium as magnesium oxide and sulfur as a sulfur dioxide gas.
In the process in Patent Literature 2, however, a process is used in which for recovering crystals of magnesium sulfate from waste water freed of a valuable substance such as nickel, the magnesium sulfate is brought into contact with concentrated sulfuric acid produced from a sulfur dioxide gas, thereby performing crystallization and dehydration. In this case, magnesium remaining without being crystallized is subjected to a leaching step again together with sulfuric acid, so that the magnesium crystallization amount depends on the amount of sulfuric acid used for leaching, and therefore it is not easy to maintain a balance. There is the problem that the degree of freedom of operations is restricted particularly when magnesium is to be separated and used as a neutralizing agent.
Patent Literature 3 suggests a leaching process for recovering nickel and cobalt from a laterite ore, the process including the steps of: separating the laterite ore into a low-magnesium-content ore fraction and a high-magnesium-content ore fraction by selective mining or post-fractionation; individually slurrying the separated ore fractions; leaching the low-magnesium-content ore fraction with concentrated sulfuric acid as a primary leaching step; and introducing a high-magnesium-content ore slurry subsequently to completion of the primary leaching step and precipitation of iron as other low-sulfur-content form of goethite, iron oxide or iron hydroxide, and leaching the high-magnesium-content ore fraction with sulfuric acid released in the iron precipitate as a secondary leaching step.
It is also considered that by using a process as described above, magnesium contained in a nickel oxide ore can be used as a neutralizing agent, or magnesium can be recovered from a neutralized solution and used as a neutralizing agent repeatedly, and as a result, leached residue capable of being provided for a low-calcium-content ironmaking raw material are obtained.
When such a process is used, however, enormous heat energy is required in concentration of magnesium from a large amount of waste water, and impurities contained in the ore are accumulated in the process as the neutralizing agent is repeatedly used.
Further, normally the grade of magnesium contained varies depending on the type, and mining location and period of an ore, and is thus unstable. Accordingly, if there is a shortage of magnesium, a conventional calcium-based neutralizing agent such as slaked lime or limestone, which is inexpensive and can be stably supplied, may be used in combination. In this case, however, calcium is brought in the process and circulated in the process system as in the case of the conventional process described above. When magnesium is to be recovered from waste water, a part of calcium exhibits the same behavior as that of magnesium to be contaminated, and therefore magnesium can no longer be used for applications other than a neutralizing agent.
As a process for separating magnesium and calcium in a solution, mention is made of, for example, a process shown in Patent Literature 4. According to the process described in Patent Literature 4, in a stack-gas desulfurization plant using magnesium hydroxide as a desulfurizing agent, magnesium hydroxide is recovered from waste liquid discarded/discharged and containing a large amount of magnesium sulfate, and circulated to a stack-gas desulfurization step to contribute to recycling and environmental cleanup.
Specifically, ammonia is added to stack-gas desulfurization waste water containing magnesium sulfate to generate and precipitate magnesium hydroxide, milk of lime is added to the left liquid to generate calcium sulfate and ammonia, and the ammonia is circulated in the process. The magnesium hydroxide thus obtained is slurried with a main process final waste liquor, and circulated to the desulfurization plant to completely circulate desulfurization plant waste water, so that waste water can be prevented from being discarded/discharged. The advantage of the resulting calcium sulfate in direct sale can be improved by providing a washing step to improve the purity thereof.
However, the process in Patent Literature 4 has the problem that since ammonia is handled, complicated equipment is required, leading to an increase in investment and operation costs, etc. and thus it is difficult to readily employ the process.
When magnesium hydroxide and magnesium oxide are produced from magnesium components contained in a nickel oxide ore, and used as a neutralizing agent as described above, an increase in costs as compared to limestone and slaked lime, and thus it is not practical to depend only on a water-soluble neutralizing agent. Further, there may be influences of calcium components etc. contained in the ore or impurities treated together.
In this connection, Patent Literature 5 describes a process for preparing magnesium oxide from a metal sulfate solution containing magnesium sulfate and calcium. In this process, metals other than magnesium are precipitated as hydroxides to separate a solid and a liquid, the separated solution is concentrated so as to have a specific gravity of 1.35 to 1.5, so that calcium sulfate is separated, and magnesium sulfate is recovered from the solution after the separation, and thermally decomposed to recover magnesium oxide.
However, the process in Patent Literature 5 has the problem that when concentration is performed for separating calcium sulfate, a part of magnesium is simultaneously precipitated together with calcium, leading to deterioration of recovery efficiency. This is because in precipitation of a compound of calcium sulfate dihydrate, precipitation of magnesium sulfate heptahydrate starts to occur in parallel, and separation of the former from the latter can be performed by various processes such as a process in which components of the solution are analyzed, a process in which a difference in appearance is observed by naked eyes, and a process in which the specific gravity is measured, but a great deal of labor and time is required.
Thus, in the conventional processes, it is not easy to inexpensively and efficiently produce magnesium oxide having a low impurity grade and a high purity, and it is difficult to stably obtain hematite, which has a low sulfur grade and calcium grade (specifically, the sulfur grade and the calcium grade are each 1% by weight or less) and is suitable as an ironmaking raw material, from a HPAL process of a nickel oxide ore using the magnesium oxide as a neutralizing agent.
Patent Literature 1: Japanese Patent Application Laid-Open No. 2005-350766
Patent Literature 2: Japanese Patent Application National Publication No. 2009-520661
Patent Literature 3: Japanese Patent Application National Publication No. 2005-523996
Patent Literature 4: Japanese Patent Application Laid-Open No. 2000-93739
Patent Literature 5: Japanese Patent Application Laid-Open No. Sho 57-500021
Patent Literature 6: Japanese Patent Application Laid-Open No. 2011-206757